In related art, in inspecting the soundness of a bond, the presence or otherwise absence of fine cracks in the bond has been checked for visually or by ultrasonic flaw detection. However, even when a skilled inspector checks a joint visually, error-free checking is difficult and also entails a lot of inspection man-hours. Although there have been attempts to evaluate the soundness of bonds using various sensors, difficulty of determination and problems of inspection accuracy have made them unsuitable for practical use.
To solve such problems, technology has been conceived in which a sensor part of an optical fiber sensor is embedded in a bond of joined members or adhered in the vicinity of the bond to assess the state of the joint of the joined members.
An optical fiber sensor is an optical fiber with a sensor part formed in its core part. The sensor part is for example a diffraction grating. An optical fiber sensor having a diffraction grating is called an optical fiber grating sensor. The construction of a sensor part is not limited to a diffraction grating. With an optical fiber sensor, a change in an optical characteristic caused by strain arising in the sensor part is used to measure the joint state. At the time of measurement using an optical fiber sensor, the sensor part of the optical fiber sensor is fixed inside an adhesive when two members are joined together by the adhesive, then light from a broadband light source is introduced into the optical fiber sensor through a light entry end thereof, and changes in reflected light or transmitted light from the sensor part are observed. From this observation, it is possible to ascertain the state of the joint between the joined members.
A measurement method of related art using an optical fiber sensor is disclosed for example in JP-A-9-101255. In this method, converting means, for example an optical fiber sensor, is disposed in the vicinity of a bonded joint. In this method, the converting means is so disposed with respect to the bonded joint that a parameter expressing a characteristic of load displacement between the joined materials and an adhesive can be recorded in correspondence with an applied load. After the assembly of the bonded joint, a reference parameter expressing a characteristic of load displacement between the joined materials and an adhesive with a reference load applied is recorded, and then a parameter expressing a characteristic of load displacement between the joined materials and the adhesive with a test load applied is recorded. The completeness of the joint is then measured by comparing the reference parameter and the parameter obtained thereafter.
Another related art technology using an optical fiber sensor is the separation inspection method of JP-A-2001-21384. In this separation inspection method, first, an optical fiber sensor is fixed to each of two members bonded together. Then, light pulses are inputted through ends of the optical fiber sensors, and strains occurring in the optical fiber sensors are measured from changes in the optical characteristics of scattered light arising in the optical fiber sensors with respect to the inputted light pulses. On the basis of differences in these measured strains of the optical fiber sensors, the occurrence of separation at the contacting faces of the members is detected.
Also, an explanation of precise measurement technology using an optical fiber sensor is set forth in Shinji Ishikawa's ‘Applied Physics’ Volume 69, Number 6 (2000), page 648 to page 654.
In the case of the method disclosed in the above-mentioned JP-A-9-101255, to check the completeness of a joint it was necessary to apply a load for testing.
In the case of the method disclosed in the above-mentioned JP-A-2001-21384, detection is difficult when the strains caused by separation are not large. The sensor part of an optical fiber sensor can detect surrounding strain as a wavelength change, but when as disclosed in JP-A-9-101255 two members are bonded using an adhesive that hardens at room temperature and the sensor part of the optical fiber sensor is embedded in that adhesive, because even when separation occurs the consequent change in strain is not large, when no load is applied the wavelength change is not large either.
For example, reflected light detection data of when the optical fiber sensor is an optical fiber grating sensor of the kind mentioned above is shown in FIG. 19(a) and (b). FIG. 19(a) is a spectrum of reflected light detected from the optical fiber sensor when separation of the joined members has not occurred. The horizontal axis is wavelength of the reflected light and the vertical axis is light strength of the reflected light. At this time, a reflected light spectrum having a peak P10 at wavelength λ1 is observed. This is because twice the period of the grating becomes an even multiple of the wavelength λ1, and reflected light from the grating at wavelength λ1 mutually enhances so that a reflected light spectrum having a peak at wavelength λ1 is observed.
On the other hand, FIG. 19(b) is a spectrum of reflected light detected from the optical fiber sensor when separation of the joined members has occurred. A small peak P11 appears at a position of wavelength λ2 near wavelength λ1, but there is not an easily distinguishable divide between the two peaks. This is because even when separation occurs, the strain occurring in the grating of the optical fiber sensor is small and the grating period does not change greatly. Because the period of the grating changes under the influence of thermal expansion when the temperature of the optical fiber sensor changes, a change occurs in the position of the peak of the reflected light spectrum when the temperature changes; consequently, the position of the peak changes when the temperature changes even when no separation has occurred, it is difficult to discern whether this is separation, and it becomes impossible to ignore the influence of temperature.
Thus, a bond separation inspection method has been awaited with which it is possible to greatly increase inspection accuracy by making a large change appear in an optical characteristic detected by an optical fiber sensor when joined members have separated.